Starship Asterisk*

by **Ann** » Sun Apr 07, 2019 4:44 pm

I forgot to credit the photographer for the stunning 8 MB Hα-rich picture of the Horsehead region. The photographer is Mark Wielgosz (2011). The technical detail that I find interesting is how much Hα has been used, and it appears to be a lot:

HaLHaRGB (4.5,4:4.5,4:4:4) Hrs

I don't see how a simple reflection nebula reflecting the blue light of a hot star could be so full of Hα. Couldn't we just as well expect the Pleiades reflection nebula to be Hα-red?

: Click to view full size image

Red Pleiades?
WISE, IRSA, NASA; Processing by Francesco Antonucci

: Click to view full size image

Red Pleiades?

Perhaps you mean that a dusty nebula, even in the presence of hot blue stars, would reflect the red light of an Hα nebula that is more distant the hot star seemingly sitting in the middle of the red and blue nebula. Can you name another such Hα reflection nebula, presuming that IC 432 is an example of one?

An alternative explanation might be that IC 432 is a reflection nebula superimposed on an emission nebula of unknown origin. But to me that is a rather contrived explanation, when an ionizing source is right there in the form of HD 37776. Bear in mind that there are other examples of stars that are ionizing an emission nebula and lighting up a reflection nebula at the same time. The most famous example is AE Aurigae.

Thanks for the brilliant photo, Mark!

Ann

by **Chris Peterson** » Sun Apr 07, 2019 4:00 pm

Ann wrote: ↑Sun Apr 07, 2019 3:54 pm
Chris Peterson wrote: ↑Sun Apr 07, 2019 1:55 pm
Nitpicker wrote: ↑Sun Apr 07, 2019 7:16 am
Funny that any galactic nebula is described as pure. It certainly looks to me like IC435 is emitting a little signal in this APOD.
Maybe, maybe not. At the least you'd need to have some photometric information, which you can't get from this processed image. The nebula may have a faint emission, normally masked by broadband reflection. But in such a bright environment as this, we could still be seeing nothing but pure reflections of the surrounding emission sources.
Certainly you may be right, and I'm aware that SIMBAD calls IC 432 a reflection nebula, but I'd like to argue my point anyway.

I'm not particularly arguing against your point, just trying to make it clear that when you have a narrowband image, it can be very ambiguous whether you're seeing emissions or reflections.

by **Ann** » Sun Apr 07, 2019 3:54 pm

Chris Peterson wrote: ↑Sun Apr 07, 2019 1:55 pm
Nitpicker wrote: ↑Sun Apr 07, 2019 7:16 am
Ann wrote: ↑Fri Apr 05, 2019 6:58 am The difference is that IC 435 is a "pure" reflection nebula, centered on B5V-type star HD 38087, which is too cool to ionize an emission nebula. IC 432, by contrast, is centered on B2IV star HD 37776, which is just hot enough to ionize a faint emission nebula. But HD 37776 is not hot and powerful enough to quickly blow away all the surrounding dust, which is why IC 432 is also a reflection nebula.

Ann
Funny that any galactic nebula is described as pure. It certainly looks to me like IC435 is emitting a little signal in this APOD.
Maybe, maybe not. At the least you'd need to have some photometric information, which you can't get from this processed image. The nebula may have a faint emission, normally masked by broadband reflection. But in such a bright environment as this, we could still be seeing nothing but pure reflections of the surrounding emission sources.

: Click to view full size image

Purple emission/reflection nebula IC 432 (center) and blue reflection nebula
IC 431 (at right, next to a white star). Source: www.deepskyastro.com

Certainly you may be right, and I'm aware that SIMBAD calls IC 432 a reflection nebula, but I'd like to argue my point anyway. The picture at right is not the only color picture that shows that IC 432 is not the same color as neighboring reflection nebula IC 431. There is a red component to IC 432 that is totally lacking in IC 431.

Other images that show the same thing are this picture by RayM0506, this small image by sky-map, as well as this image by Jim Janusz.

Personally I find this 8 MB Hα-rich portrait of the Horsehead region to be particularly interesting. There can be no doubt that the B2IV-type star is sitting in the middle of a Hα nebula, which appears to be quite separate from other Hα nebulas in the field.

So if it looks like a duck, and quacks like a duck, should we then assume that it is a duck? Or if it is a hot star sitting in the middle of what appears to be an emission nebula, should we assume that the hot star is the ionizing source of the nebula?

Occam would say that we should.

Ann

by **Chris Peterson** » Sun Apr 07, 2019 1:55 pm

Nitpicker wrote: ↑Sun Apr 07, 2019 7:16 am
Ann wrote: ↑Fri Apr 05, 2019 6:58 am The difference is that IC 435 is a "pure" reflection nebula, centered on B5V-type star HD 38087, which is too cool to ionize an emission nebula. IC 432, by contrast, is centered on B2IV star HD 37776, which is just hot enough to ionize a faint emission nebula. But HD 37776 is not hot and powerful enough to quickly blow away all the surrounding dust, which is why IC 432 is also a reflection nebula.

Ann
Funny that any galactic nebula is described as pure. It certainly looks to me like IC435 is emitting a little signal in this APOD.

Maybe, maybe not. At the least you'd need to have some photometric information, which you can't get from this processed image. The nebula may have a faint emission, normally masked by broadband reflection. But in such a bright environment as this, we could still be seeing nothing but pure reflections of the surrounding emission sources.

by **Nitpicker** » Sun Apr 07, 2019 7:16 am

Ann wrote: ↑Fri Apr 05, 2019 6:58 am The difference is that IC 435 is a "pure" reflection nebula, centered on B5V-type star HD 38087, which is too cool to ionize an emission nebula. IC 432, by contrast, is centered on B2IV star HD 37776, which is just hot enough to ionize a faint emission nebula. But HD 37776 is not hot and powerful enough to quickly blow away all the surrounding dust, which is why IC 432 is also a reflection nebula.

Ann

Thanks Ann. Funny that any galactic nebula is described as pure. It certainly looks to me like IC435 is emitting a little signal in this APOD.

I wonder what proportion of gas and dust lurks in the galaxy, undetected by science?

by **Ann** » Fri Apr 05, 2019 6:58 am

Nitpicker wrote: ↑Fri Apr 05, 2019 6:05 am
Ann wrote: ↑Fri Apr 05, 2019 5:40 am My bad. The nebula is, of course, IC 432. Forget everything I said about IC 435.

Ann
Don't be too hard on yourself, Ann, they are just funny coloured puffs of almost nothing, after all. I was also unsure which was which when I was writing my earlier posts, so I only referred to the image colours in general.

But both puffs are said to be reflection nebulae, and I suppose the interesting thing about them, in this context, is that their reflected starlight is largely filtered out from the narrowband APOD, revealing the elemental nature of the puffs themselves.

The difference is that IC 435 is a "pure" reflection nebula, centered on B5V-type star HD 38087, which is too cool to ionize an emission nebula. IC 432, by contrast, is centered on B2IV star HD 37776, which is just hot enough to ionize a faint emission nebula. But HD 37776 is not hot and powerful enough to quickly blow away all the surrounding dust, which is why IC 432 is also a reflection nebula.

Ann

by **Nitpicker** » Fri Apr 05, 2019 6:05 am

Ann wrote: ↑Fri Apr 05, 2019 5:40 am My bad. The nebula is, of course, IC 432. Forget everything I said about IC 435.

Ann

Don't be too hard on yourself, Ann, they are just funny coloured puffs of almost nothing, after all. I was also unsure which was which when I was writing my earlier posts, so I only referred to the image colours in general.

But both puffs are said to be reflection nebulae, and I suppose the interesting thing about them, in this context, is that their reflected starlight is largely filtered out from the narrowband APOD, revealing the elemental nature of the puffs themselves.

by **Ann** » Fri Apr 05, 2019 5:40 am

Ann wrote: ↑Thu Apr 04, 2019 5:22 am Let's compare the appearance of IC 435 in Hα, OIII and SII (the APOD) with an RGB image.

Ann

My bad. The nebula is, of course, IC 432. Forget everything I said about IC 435.

Ann

by **Chris Peterson** » Fri Apr 05, 2019 3:37 am

Nitpicker wrote: ↑Thu Apr 04, 2019 7:46 pm
Chris Peterson wrote: ↑Thu Apr 04, 2019 1:22 pm
Nitpicker wrote: ↑Thu Apr 04, 2019 7:18 am What I am less sure about is the reasoning behind the different exposure times for each channel. If I attempt to account for the 2x2 binning in the G and B channels, it looks like the effective ratios from each channel are about R=10%, G=60%, B=30%. I am sure there must be a reason to do this, but I don't know how one would decide on a basis.
Keep in mind that the exposure time used to collect the data does not necessarily correspond to the strength of the signal in final image. I normally set my exposure time based on the S/N level I desire in that data channel, and then weight the channels separately in the final image.
Thanks Chris. That does make sense. So, on what basis should one decide to weight the channels? Is this APOD representative of the relative strengths of signal recorded in each channel? Given that the SII is seemingly so overpowered by the other two channels, I am wondering whether showing it amplified might be more informative. As it is, I can't really tell where the sulfur is.

I think there are good reasons to maintain photometric accuracy (that is, where the intensity of each output channel corresponds to the actual intensity of the signal mapped to that channel, regardless of the exposure time used), and also for deliberately changing the photometric ratios between the channels (e.g. boosting the SII signal). If it were me, I'd probably try different things with the data and see what showed the most interesting or richest structure and detail.

by **Nitpicker** » Fri Apr 05, 2019 12:43 am

Ann wrote: ↑Thu Apr 04, 2019 7:05 pm What I don't understand about today's APOD is the blue mapping of the reddest channel. But maybe that was for aesthetic reasons, or in order to map Hα as red and OIII as green.

Ann

The APOD is just "RGB=HOS", rather than the "RGB=SHO" that is perhaps more commonly seen in narrowband images, and which is sometimes referred to as the Hubble palette for narrowband images, I think.

Some might argue that with the predominance of hydrogen in the universe, it makes more sense to map Ha, and not SII, to the red channel. And the colour of OII wavelengths is arguably closer to green than blue. So, that leaves SII to be mapped to the blue channel. Simple.

And with the blue channel being overpowered by the others in this APOD, the combination is probably reasonably close to the true colours of the wavelengths passing through the filters. (But I can well imagine that lovers of blue might not like it as much.)

by **Nitpicker** » Thu Apr 04, 2019 7:46 pm

Chris Peterson wrote: ↑Thu Apr 04, 2019 1:22 pm
Nitpicker wrote: ↑Thu Apr 04, 2019 7:18 am What I am less sure about is the reasoning behind the different exposure times for each channel. If I attempt to account for the 2x2 binning in the G and B channels, it looks like the effective ratios from each channel are about R=10%, G=60%, B=30%. I am sure there must be a reason to do this, but I don't know how one would decide on a basis.
Keep in mind that the exposure time used to collect the data does not necessarily correspond to the strength of the signal in final image. I normally set my exposure time based on the S/N level I desire in that data channel, and then weight the channels separately in the final image.

Thanks Chris. That does make sense. So, on what basis should one decide to weight the channels? Is this APOD representative of the relative strengths of signal recorded in each channel? Given that the SII is seemingly so overpowered by the other two channels, I am wondering whether showing it amplified might be more informative. As it is, I can't really tell where the sulfur is.

by **Ann** » Thu Apr 04, 2019 7:05 pm

Chris Peterson wrote: ↑Thu Apr 04, 2019 4:08 pm
Ann wrote: ↑Thu Apr 04, 2019 3:28 pm Personally I would love to see astrophotographers, both amateurs and professionals, explain not only what filters they used, but also why they chose those particular filters.
As a rule, images made in the visible spectrum are either involve broadband RGB filters, in which case the intent is "true color" (although processing can significantly impact the final result, since there's really no absolute "true" when it comes to color), or they are using narrowband filters, in which case the intent is generally pretty obvious simply from the choice of filters.

What I don't understand about today's APOD is the blue mapping of the reddest channel. But maybe that was for aesthetic reasons, or in order to map Hα as red and OIII as green.

Ann

by **Chris Peterson** » Thu Apr 04, 2019 4:08 pm

Ann wrote: ↑Thu Apr 04, 2019 3:28 pm Personally I would love to see astrophotographers, both amateurs and professionals, explain not only what filters they used, but also why they chose those particular filters.

As a rule, images made in the visible spectrum are either involve broadband RGB filters, in which case the intent is "true color" (although processing can significantly impact the final result, since there's really no absolute "true" when it comes to color), or they are using narrowband filters, in which case the intent is generally pretty obvious simply from the choice of filters.

by **Ann** » Thu Apr 04, 2019 3:28 pm

Nitpicker wrote: ↑Thu Apr 04, 2019 7:18 am The red/orange/brown/yellow shades in the APOD are showing the dominance of the red channel (Ha ~656nm [red]) and green channel (OIII ~500nm [cyan-green]), with a relative lack of signal in the blue channel (SII ~672nm [the reddest wavelengths recorded]). The yellow shades are where the the signal from the oxygen reaches parity with the hydrogen.

It is true that no blue wavelengths were recorded for this APOD, but that's not why there is less blue in it. It is less blue because there is less signal from the sulfur. SII and Ha are very close in wavelength, both very red. The APOD has separated them, where a "true colour" broadband image cannot.

: Click to view full size image

The Horsehead region in different wavelengths.
Photo: ESA Science.

The Horsehead region contains a lot of intrinsically shortwave light (~450 nm and shorter), but the filters used for the APOD were not able to detect much of this blue light. That is not wrong in itself, of course. But it does mean that the portrait of the Horsehead region will look different than it does in an RGB image.

The pictures at right show the Horsehead region in different wavelengths. Each picture is "true", but they do look different.

Personally I would love to see astrophotographers, both amateurs and professionals, explain not only what filters they used, but also why they chose those particular filters. What did they want their picture to show us? And if they "color-mapped" their filter exposures in an unusual way, was there a reason for that? Maybe just aesthetics?

I would love to know. Because different filters and different mappings do affect what we see in the picture.

Ann

by **Chris Peterson** » Thu Apr 04, 2019 1:22 pm

Nitpicker wrote: ↑Thu Apr 04, 2019 7:18 am What I am less sure about is the reasoning behind the different exposure times for each channel. If I attempt to account for the 2x2 binning in the G and B channels, it looks like the effective ratios from each channel are about R=10%, G=60%, B=30%. I am sure there must be a reason to do this, but I don't know how one would decide on a basis.

Keep in mind that the exposure time used to collect the data does not necessarily correspond to the strength of the signal in final image. I normally set my exposure time based on the S/N level I desire in that data channel, and then weight the channels separately in the final image.

by **Nitpicker** » Thu Apr 04, 2019 7:18 am

The red/orange/brown/yellow shades in the APOD are showing the dominance of the red channel (Ha ~656nm [red]) and green channel (OIII ~500nm [cyan-green]), with a relative lack of signal in the blue channel (SII ~672nm [the reddest wavelengths recorded]). The yellow shades are where the the signal from the oxygen reaches parity with the hydrogen.

It is true that no blue wavelengths were recorded for this APOD, but that's not why there is less blue in it. It is less blue because there is less signal from the sulfur. SII and Ha are very close in wavelength, both very red. The APOD has separated them, where a "true colour" broadband image cannot.

What I am less sure about is the reasoning behind the different exposure times for each channel. If I attempt to account for the 2x2 binning in the G and B channels, it looks like the effective ratios from each channel are about R=10%, G=60%, B=30%. I am sure there must be a reason to do this, but I don't know how one would decide on a basis.

by **Ann** » Thu Apr 04, 2019 5:22 am

Let's compare the appearance of IC 435 in Hα, OIII and SII (the APOD) with an RGB image.

: Click to view full size image

Pale orange IC 435 in Hα, OIII and SII.

: Click to view full size image

Red and blue IC 435 (at upper left) in RGB.

I recommend this much larger RGB image by Paddy Gilliland FRAS where you can see IC 435 well. Note that IC 435 (at upper left) looks like a red patch of Hα light overlaid by streaks of blue reflection nebulosity.

Note that the shape of IC 435 in the APOD. Note that it corresponds well to the patch of red nebulosity in the RGB images. That is because the filters used for the APOD strongly detects red Hα light, but it is rather bad at detecting blue broadband light. All it can see of blue light is the blue-green OIII line at 501 nm. That is why IC 435 looks orange in the APOD, as a result of a lot of red Hα and a little bit of green OIII.

Note that the ionizing star of IC 435, HD 37776, shows up quite poorly in the APOD. Since HD 37776 is a hot star dominated by ultraviolet and blue light, the red and green filters of the APOD does not detect it very well. (Though I assume that the processing might have something to do with it, too.)

The reason why I wrote this post is to explain why IC 435 looks rather different in today's APOD than it does in most pictures of the Horsehead nebula region.

Ann

by **Nitpicker** » Thu Apr 04, 2019 4:35 am

But it is one of the best approximations to what our eyes might see, if they were more sensitive and had narrowband filters. If ifs and buts were sweets and nuts, we'd all have a merry solstice.

by **Ann** » Thu Apr 04, 2019 1:53 am

orin stepanek wrote: ↑Thu Apr 04, 2019 1:22 am Thanks guys; i appreciate your answers! I just didn't recall seeing that on previous photos of the area!

: Click to view full size image

Horsehead and Orion Nebula region.
Photo: Terry Hancock.

Today's APOD looks different in many respects because its color scheme is not a close approximation of what the human eye would see, if our eyes were many times more sensitive than they are. The picture of the Horsehead region and the Orion nebula by Terry Hancock is an RGB image, and therefore it corresponds a lot more closely to what the color-sensitive cones in our retinas would see, if, as I said, our eyes were many times more sensitive.

Note the tiny black protrusion in Terry Hancock's image, which is the Horsehead nebula. To the left of the Horsehead is a smallish orange "cloud" crossed by a strong black "band". That is the Flame nebula. Above the Flame nebula is Alnitak, and to the lower left of Alnitak is IC 435, which is a combined blue reflection and red emission nebula. Its color in Terry Hancock's image is therefore slightly purplish.

Ann

by **orin stepanek** » Thu Apr 04, 2019 1:22 am

Thanks guys; i appreciate your answers! I just didn't recall seeing that on previous photos of the area!

by **Chris Peterson** » Wed Apr 03, 2019 11:57 pm

orin stepanek wrote: ↑Wed Apr 03, 2019 10:44 am I love the photo's color scheme; nicely done! What is the large ball to the left of the Horse's head? It must be huge! :shock:

IC 432. A reflection nebula.

by **Chris Peterson** » Wed Apr 03, 2019 11:56 pm

Indigo_Sunrise wrote: ↑Wed Apr 03, 2019 11:02 am Gorgeous image! 8-)

Why does it appear like there is a 'bubble' around Alnilam..? Is it possibly a 'side effect' of processing? Or perhaps it's noticeable in other images of that star?
*offtolookforcomparisons*

An artifact, I think. The star is thousands of times brighter than the nebulosity around it, and in the long exposures required for the latter would have resulted in a massive bloated blob. The bubble effect is probably what's left from trying to edit that out.

by **neufer** » Wed Apr 03, 2019 11:36 pm

orin stepanek wrote: ↑Wed Apr 03, 2019 9:42 pm

Yeah...that's what I assumed with my answer above.

HD 37776

IC 432

IC 435

by **Boomer12k** » Wed Apr 03, 2019 9:45 pm

Awesomely colored image...looks like it was painted almost...amazing detail...

:---[===] *

by **orin stepanek** » Wed Apr 03, 2019 9:42 pm

Starship Asterisk*

APOD: Wisps Surrounding the Horsehead Nebula (2019 Apr 03)

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